Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Please remove the battery from the device.
(How old is the Harman Becker Map Pilot you have?)
That's done by pressing no the sliding battery cover and pushing it in the direction of the arrow.
Look on the battery label. What is its part #, please? that isn't listed anywhere that I can find.
With that information (and maybe the voltage, or 'V', which will be listed on the battery's label also) I can find a source for the replacement.